Add VLANs to network interface?

I need to add 2 VLANs to a network interface on FreePBX. I’ve tried adding virtual interfaces as the instructions suggest (eth0:20, eth0:30), but neither will allow me to interface with the VLANs. I have also tried eth0.20, eth0.30, but those do not work, either.
What I found that does work is adding lines to the /etc/sysconfig/network-scripts/eth0.xx files that say “VLAN=yes”. I have not tried doing this while using the suggested “:” separator - is there a reason this is a colon, and not a period? Would I be better off not adding the ‘virtual interfaces’ in FreePBX, and just creating the ifcfg files from scratch?